In recent times, fabrication of integrated circuits has resulted in extremely dense circuits For example, such densely integrated circuits have feature sizes on the order of 1 micron This density is a direct result of achieving high resolution masks through projection photolithography during the fatrication process.
In prior art projection photolithography, an image, that is, a preselected pattern, is projected onto a layer of photoresist, and the exposed portions of the photoresist layer absorbs the energy of the projected image. The photoresist layer then is chemically developed and processed to form a mask of the projection image, and the mask is used in subsequent etching processes. If the image projected contains extremely small, sharply defined features, the mask produced also defines small features, and the integrated circuits fabricated from the mask can be made correspondingly dense. Unfortunately, two phenomena in the early prior art act to limit the smallness of features and the sharpness of the projection image: standing waves and nonuniform topology of the photoresist layer. The disadvantages of these phenomena have been discussed in detail by Tai et al. in "Submicron optical lithography using an inorganic resist/polymer bilevel scheme," J. Vac. Sci. Technol., 17(5), Sept./Oct. 1980, pp. 1169-1177. These disadvantages are now summarized.
In the early prior art photolithography photoresist layer, light waves are reflected at the surfaces of the layer to interfere with incident waves. The interference between incident and reflected waves causes standing waves within the layer, and the nodes of the standing waves become sites of minimal energy absorption to cause differing light exposure or dose within the layer. As a result, the mask pattern created from the photoresist layer does not sharply correspond to the projection image. The resolution of the image in the mask consequently becomes limited in part by the extent of such standing waves.
In forming a layer of photoresist, the profile, or topography, of the layer is generally not uniform. In other words, the layer normally has, microscopically speaking, undulations of valleys and peaks to result in a layer of varying thickness. Because of the varying thickness, the absorption of energy when an image is projected will also not be uniform, and the image developed in the photolithography mask will correspondingly be not uniformly sharp. The resolution of the mask consequently also becomes limited by the extent of nonuniformity of the photoresist layer topography.
To counter these limiting effects of the early prior art photolithography process, the method represented by FIG. 1 and disclosed by Barlett et al. in "A Two Layer Photoresist Process in a Production Environment," Proceedings of SPIE, Vol. 394, 1983, pp. 49-56, is used. In this prior art process, double thickness layers 12 are formed by applying a dye-photoresist layer 22 to an integrated circuit substrate 10 and then superposing a photoresist layer 33 on it. The dye-photoresist layer 22 typically contains a mixture of a dye to absorb reflection waves and in that way eliminate or minimize the standing wave effect. Furthermore, a double thickness layer 12 tends to distribute more uniformly over the integrated circuit substrate 10; this in turn reduces the non-uniform topology effect. In this double thickness layer process, the photoresist layer 33 is exposed and developed to provide a relatively planar mask for patterning the underlying dye-photoresist layer 22. The close proximity of the mask to the dye-photoresist layer in combination with the reflection absorbing dye allows creating a mask of enhanced resolution from the dye-photoresist layer 22 for use in subsequent etching. This process, however, has an inherent disadvantage: control of line width is very difficult because of the high sensitivity of the process to any variations in image exposure dose and photoresist development time and temperature. Consequently, the actual dimensions realized in the process vary considerably from the intended projection dimensions to result in integrated circuits having much less resolution than the projection circuits.
West et al. in "Contrast Enhancement--A Route to Submicron Optical Lithography," Proceedings of SPIE, Vol. 394, 1983, pp. 33-38, disclose another refinement in the prior art to overcome the disadvantages in early prior art photolithography for improving the resolution in the photolithographic process. West et al. teach that a photobleachable layer is formed over a photoresist layer. An image is then bleached into the photobleachable layer by irradiating the photobleachable layer with a light of high intensity. As the photobleachable layer is bleached and becomes progressively a transparent optical mask, the irradiating light continues through the transparent optical mask and simultaneously exposes the photoresist layer. In other words, this method creates an optical mask capable of high resolution to be used for exposing a photoresist sublayer. But this prior art scheme requires that the light source remains sufficiently intense in one area to both photobleach the photobleachable layer into an optical mask and expose the photoresist layer with sufficient light to allow the photoresist layer to be developed later but not to further degrade the lightsensitive optical mask. Herein lies its disadvantage: this prior art process, though an improvement over the early prior art, presents a conflicting requirement of irradiating an optical mask and illuminating a photoresist mask with a common light source. This process, then, requires a careful control of light dose in the process so that only a dose sufficient to expose a photoresist layer throught an optical mask is used; otherwise, the optical mask is further degraded and resolution in the integrated circuit fabrication process is lost.